This invention concerns a vibration damping soundproof sheet used for forming a vibratio damping soundproof structure comprising a vibration damping layer secured to the panel surface of a vehicle (particularly, floor surface) and a sound proof layer formed on the vibration damping layer.
As shown in FIG. 1, the vibration damping soundproof structure as described above comprises a vibration damping layer 2 prepared from a sheet-like vibration damping layer-forming material made of rubber type adhesive blend by setting the sheet to the floor surface of a vehicle and then heating to soften the material during passage through a drying furnace or the like thereby fitting to the floor surface of a vehicle (vehicle panel) 1, and a soundproof layer 3 made of a soundproof layer-forming material made of felt (of poor fitness) by cutting patching the material on the vibration damping layer 2. A mat layer-forming material (usually lined carpet) is bonded on the soundproof layer 3 to form a mat layer 5. The felt materials used herein are those prepared from natural fibers, synthetic fibers, regenerated fibers, etc impregnated with synthetic resin such as phenol resin or vinyl acetate resin into a plate-like configuration.
However, the vibration damping soundproof structure as described above involves the following problems:
(1) Since the vibration damping layer and the soundproof layer are formed by different steps respectively and the soundproof layer is formed by patching work, the number of working steps is increased as a whole.
(2) Since the felt material is applied by patching, the structure partially includes those portions not formed with the soundproof layer as illustrated in the drawing failing to provide satisfactory soundproofing effect.
In order to overcome the foregoing problems, the present inventors have already proposed a vibration damping soundproof sheet for use in vehicles comprising a vibration damping layer-forming material made of a rubber type adhesive blend and a soundproof layer forming-material made of a foamed rubber blend appended and laminated to each other.
The vibration damping soundproof sheet is applied by setting the material on the panel surface of a vehicle and passing through a drying furnace (usually at a heating temperature from 40.degree. to 160.degree. C.). Then, as shown in FIG. 2, the damping layer-forming material 7 is heated to soften and fitted to the panel surface 1 of the vehicle by its own weight as shown in FIG. 3 and the soundproof layer-forming material 8 is foamed and they constitute a vibration damping layer 12 and a soundproof layer 13 respectively.
Although the foregoing problems can be solved by the vibration damping soundproof sheet, if there is unevenness on the panel surface 1 of the vehicle, the soundproof layer-forming material 8 together with the vibration damping layer-forming material 7 are fitted along the configuration of the vehicle panel surface as shown in FIG. 3 to form an uneven shape to the surface of the soundproof layer 13. However, this is not desirable in view of the increasing users' demand for making the floor surface flat in recent years.
Further, since the porous soundproof layer 13 is formed to the inside of the chamber of the vehicle, there is an additional problem that the soundproofing effect can not be obtained effectively for these sounds coming from the outsides that should be obtained by pores of the porous soundproof layer 13, that is, attenuation of air vibrations and conversion into heat energy for acoustic waves from a sound source (sounds coming out of the vehicle) due to the resistance at the circumferential walls of the pores when the waves intrude therein.
Accordingly, although it is desired to increase the thickness of the porous soundproof layer in order to improve the sound-barrier property, it is undesirable resulting in increase in the weight and the material cost (refer to Comparative Example in Table 6).